The Trump administration is seeking to allow US authorities to keep immigrant children in detention longer than the current 20-day limit, a move critics denounced as "sickening" and "cruel".

The US government said on Thursday it plans to abandon a long-standing federal court agreement that strictly defines the conditions under which minors can be held in immigration detention facilities.

"Today, legal loopholes significantly hinder the ... ability to appropriately detain and promptly remove family units that have no legal basis to remain in the country," Secretary of Homeland Security Kirstjen Nielsen said.

The announcement also proposed new rules that would allow US authorities to detain minors for the duration of their families' immigration proceedings.

The regulations aim to end the Flores settlement, a 1997 court ruling that limits the length of time minors can be detained to 20 days and sets out the conditions that must be afforded to them.

The agreement requires the government release children to their parents, relatives or qualified programmes "without unnecessary delay" and to keep children in the "least restrictive conditions" possible while in custody, including separating them from adult detainees. In 2015, the agreement was expanded to cover children detained while with their parents.

Thousands separated

The Flores agreement became an issue earlier this year when the Trump administration adopted a "zero-tolerance policy" of prosecuting anyone caught crossing illegally.

More than 2,900 children were separated from their parents and held in harsh conditions, prompting international outrage and leading a federal judge to order reunification of the families in June.

More than 500 children were still separated from their parents in late August, according to reports.

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The Trump administration plans to publish new regulations in the Federal Register on Friday, which will attempt to replace the Flores settlement.

The proposed regulations will also allow the Immigration and Customs Enforcement agency to detain children in facilities not falling under state licensing regulations, a publicly available copy said.

'Trauma for children'

Representative Paul Ryan, the highest-ranking Republican in the US House of Representatives, said he "just heard about it [proposed regulations] this morning".

"My position's been really clear about this: we should not be separating people at the border. I believe that the administration also agrees," Ryan said.

The move angered immigrant rights activists and is likely to trigger another round of court battles.

"It is sickening to see the United States government looking for ways to jail more children for longer," said Omar Jadwat, director of the American Civil Liberties Union’s Immigrants' Rights Project.

"And it's yet another example of the Trump administration's hostility toward immigrants resulting in a policy incompatible with the most basic human values."

Jadwat accused the administration of "trying to expand the trauma it is inflicting on these children in order to deter other people from coming to the country".

Rachel Prandini, staff attorney at the Immigrant Legal Resource Center, told The Associated Press the erosion of Flores' protections would subject children to worsening conditions.

"The Trump administration's decision to exacerbate the suffering of kids - by imposing the cruel policy of family separation earlier this summer and now with this rule change to vastly expand detention of children - is horrifying," she said.